The manufacture of thin film circuit devices involves the etching of gold and palladium from substrates with an aqueous potassium iodide-iodine etching solution, to form thin film circuit patterns on the substrates. As gold is dissolved into the etching solution, the etching solution becomes spent and the etching rate of the solution decreases to a point where the solution is no longer effective in a manufacturing process.
It then is desirable to remove the expensive gold and palladium metals from the spent etching solution for reuse. Preferably, this should be accomplished in a manner which does not contaminate the etching solution, so that the etching solution also can be regenerated for reuse after the gold and palladium have been removed. Accordingly, a purpose of this invention is to provide a relatively simple and economical method of recovering gold from a spent aqueous potassium iodide-iodine etching solution and then regenerating the spent etching solution to substantially its original strength.
An evaporation-type process for recovering a spent aqueous potassium iodide-iodine solution which has been used for the etching of gold, is disclosed on pages 23 and 24 of the January, 1977 issue of the Western Electric Technical Digest No. 45. In this process, a metered charge of the spent etching solution is transferred into a sealed heat chamber containing an inert atmosphere (carbon dioxide) to prevent oxidation. The charge of spent etching solution is then heated so that water vapor is driven from the aqueous solution and passes through a heated conduit to a relatively cool potassium iodide solution, in which the water vapor is condensed. As the water in the spent etching solution distills, some unreacted iodine also is carried over into the potassium iodide solution. Driving off of the water from the spent etching solution leaves a powder residue of potassium iodoaurate, unreacted iodine and potassium iodide. Continued application of heat then causes a thermal decomposition of the powder, causing the unreacted iodine to be driven off as a gas into the potassium iodide solution, in which it becomes dissolved to produce a potassium iodide-iodine etching solution. As the heating continues, the potassium iodoaurate is decomposed to produce a solid dry mixture of gold and potassium iodide, which then is placed in water to dissolve the potassium iodide, leaving the gold in a free metallic state. The gold is then recovered from this solution by filtering and the filtered potassium iodide solution may be used to collect the iodine gas during the heating process, thus regenerating the original etching solution for reuse.
The Homick et al. U.S. Pat. No. 3,957,505 discloses a process for reclaiming gold in which dissolved gold in an aqueous potassium iodide-iodine solution is precipitated from the solution by the addition of hydrazine and sodium hydroxide. After removal of the precipitated gold from the solution, the solution is restored to its original condition by the addition of hydrogen peroxide.
The Wilson U.S. Pat. No. 3,709,681 is directed to a process for the recovery of noble metals utilizing a solvent comprising diacetone alcohol as the major component, water, and minor amounts of glacial acetic acid, potassium iodide and elemental iodine. Solvation of the noble metal occurs during heating and agitation of the resultant slurry and removal of the noble metal is achieved by displacement thereof onto a non-noble metal surface. Aqueous hydroxide then is used to convert any excess non-noble metal to its water-soluble salt. The remaining insoluble material then is rinsed to remove any remaining unreacted alkali and the water-soluble salts, and is digested with concentrated sulfuric acid to dissolve any remaining and soluble impurities. The remaining acid insoluble residue, which is then rinsed and dried, comprises substantially pure noble metal. A similar process, for the recovery of gold in an iodine-containing solvent, also is disclosed in the Wilson U.S. Pat. No. 3,778,252.
It is known that when an aqueous iodine solution is made basic, it undergoes a hypohalite reaction in which the iodine is consumed, producing hypoiodite, iodide and water.